JP3154288B2 - Method of correcting thermal displacement of ball screw in NC machine tool - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calculating and estimating the amount of thermal displacement of a ball screw in an NC machine tool from the operating state thereof and automatically correcting the pitch error correction function of an NC controller. .

[0002]

2. Description of the Related Art Conventionally, methods for compensating for the thermal displacement of a ball screw are roughly classified into a method for suppressing the thermal displacement of the ball screw itself, a method for measuring the thermal displacement of the ball screw, and NC control based on the measurement result. The method of modifying the pitch error correction function of the device has been mainly adopted. The former, by fixing supporting both ends in a so-called double anchor bearing while applying a tensile force to it <br/> Ji of the screw shaft ball, how to absorb the pretension thermal expansion of the ball screw, the ball There is a method of forcibly cooling the screw with oil or air (usually by providing a cooling through hole in the screw shaft). In the latter case, the amount of thermal displacement of the ball screw is directly measured by a measuring device such as a linear scale or displacement sensor to make corrections.The amount of temperature rise of the ball screw is measured with a thermocouple, etc. There is a method of calculating the amount and making a correction.

[0003]

By the way, recently, NC
Large diameter ball screws are often used in machine tools, and are often operated at high speed. Therefore, when trying to absorb the thermal displacement by the pretension of the screw shaft, a very large tensile force must be applied to the screw shaft, which may cause a deformation of the structure and a reduction in the life of the support bearing. Further, if thermal displacement is to be suppressed by forced cooling, a special structure corresponding to the displacement is required, and the structure becomes complicated and large. Further, the correction by measurement requires hardware such as an instrument and a device for measurement, which leads to an increase in cost. Moreover, since external factors such as hardware measurement accuracy and measurement failure are directly reflected in the correction amount, it is often difficult to ensure reliability.

Accordingly, an object of the present invention is to provide a correction method capable of easily and accurately correcting the thermal displacement of a ball screw without using the special structure or hardware as described above. It is assumed that.

[0005]

SUMMARY OF THE INVENTION A thermal displacement correcting method according to the present invention is directed to a method of correcting a rotational speed and a time obtained from a temperature rise curve of a ball screw.
The curve representing the relationship with the constant is divided into a plurality of rotation speed
Linear approximation of this curve for each rotation speed region
(Τ) is obtained, and the ratio (q) of the number of rotations (N) of the motor per sampling unit time (ΔTs) in the control unit time (Ts) to the time the motor rotates within the control unit time (Ts) : Operating time ratio), the calorific value (Q) of the ball screw is obtained, and the calorific value (Q) and the motor
Using the above time constant (τ) corresponding to the rotation speed (N), the estimated value (ΔL) of the thermal displacement of the ball screw from the first-order lag equation
Is determined, a correction area to be corrected for the pitch error correction amount of the memory-type pitch error correction function of the NC control device is set, and the estimated value (ΔL) is set to a negative or positive area boundary point of the correction area. And corrects the pitch error correction amount by linearly and uniformly distributing and adding it to each point in the correction area.

[0006]

[0007]

If the operation of the machine is interrupted, the time (t) at which the correction control of the pitch error correction amount is performed is stored every moment, and the power-off time (from the previous power-off time to the present power-on time) is stored. T) is compared with the heat radiation time constant (τ ₀ ) obtained from the heat radiation curve of the ball screw. In the case of (T> τ ₀ ), the process of calculating the above-mentioned estimated value (ΔL) is performed using the current power-on time as the initial time. In the case of (T <τ ₀ ), it is preferable to perform the calculation processing of the estimated value (ΔL) with the previous power-on time as the initial time again.

When considering the effects of the initial temperature and the ambient temperature, it is preferable to use a measurement cycle together . Sand
In addition, the initial length of the ball screw is
The estimated value (ΔL) that has been measured and
Cancel and renew based on the above initial length
The arithmetic processing of the estimated value (ΔL) is performed.

[0010]

[Function] Since the amount of thermal displacement of the ball screw is estimated from the rotation speed of the motor and the operation time ratio, there is no need to use special hardware when calculating the amount of thermal displacement, and the thermal displacement is corrected by simple and quick arithmetic processing. Can be performed. In addition, a correction area to be corrected for the pitch error correction amount is set, and an estimated value of the thermal displacement amount is given to either the negative or positive side area boundary point of the correction area, and this is set in the correction area. Are linearly and uniformly distributed and added to each point, so that the arithmetic processing for correcting the thermal displacement can be further simplified and speeded up.

The time constant τ in the equation of the first-order lag is obtained by dividing a curve representing the relationship between the rotation speed and the time constant obtained from the temperature rise curve of the ball screw into a plurality of rotation speed regions, and setting the curve for each rotation speed region. Can be obtained with relatively high accuracy without performing a complicated analysis operation.

When the operation of the machine is interrupted, the power-off time from the previous power-off time to the current power-on time is compared with the heat radiation time constant obtained from the radiation curve of the ball screw, and the cold start and the hot start are determined. Correction accuracy can be improved by performing the computation process separately.

When considering the effects of the initial temperature and the ambient temperature, the above-mentioned estimated value (ΔL) is calculated based on the initial length of the ball screw measured by the measurement cycle. Thereby, the correction accuracy can be improved.

[0014]

Embodiments of the present invention will be described below.

First, the axial thermal displacement ΔL of the ball screw
Can be expressed by the following equation: ΔL = L · ρ · θ Equation L: Overall length of the ball screw ρ: Thermal expansion coefficient of the ball screw θ: Temperature rise The temperature rise of the ball screw θ is the following primary It can be expressed by a delay equation.

Θ = Q {1−exp (−t / τ)} Equation Q: Heat value t: Time from start τ: Time constant Then, the following first-order lag equation is derived from the equation and the equation. Can be.

ΔL = L · ρ · Q {1−exp (−t / τ)} Equation (1) Here, the heat generation amount Q is estimated from the rotational speed N of the motor driving the ball screw by the following expression.

Q = K · N · q Equation K: Coefficient (determined by experiment) N: Number of rotations per sampling unit time ΔTs, rotation of motor within control unit time Ts (cycle time of correction operation) Average number

Q: Operating time ratio within control unit time Ts (T
The ratio of the time during which the motor is rotating within s)

N and q in the equation are obtained as follows. For example, in the control unit time Ts, the motor
If you rotate at the rotation speed shown in

Average number of rotations N = (4 + 4 + 4 + 4 + 2)
+ 2 + 2 + 2) / 8 = 3 Operating time ratio q = 8/10 = 0.8 N · q = 3 × 0.8 = 2.4 For example, assuming that the motor rotates at a rotational speed as shown in FIG. Average value N = (2 + 2 + 2 + 1) /4=1.75 Operating time ratio q = 4/10 = 0.4 N · q = 1.75 × 0.4 = 0.7 Note that the control unit time TS and the sampling unit time ΔTs may be arbitrarily determined, but in FIGS. 1A and 1B, the control unit time Ts is set to about 10 seconds, and this is set to 10 sampling unit times ΔTS (about 1 second). It is divided. From the (N · q) obtained in this way, the calorific value Q is obtained from the equation.

Next, the time constant τ in the equation of the first-order lag
Is obtained from the result of the experiment as follows. The time constant τ is different between when heat is generated (when the machine power is turned on: q> 0) and when heat is radiated (when the machine power is turned off: q = 0). However, in the case of heat generation, the value differs depending on the number of revolutions, so that the same approximation method as in the case of heat radiation cannot be adopted.
Of course, it is possible to analyze and calculate the time constant τ as a function of the time and the rotation speed from the temperature rise curve of the ball screw obtained by the experiment, but it requires a very complicated calculation process. Therefore, in this embodiment, a curve (determined by an experiment) representing the relationship between the rotation speed and the time constant obtained from the temperature rise curve of the ball screw is divided into a plurality of rotation speed regions. Is linearly approximated to obtain a time constant τ. For example, a curve representing the relationship between the rotation speed and the time constant is divided into three rotation speed regions on the basis of two rotation speed thresholds (H, M: H> M) (N ≧ H, H> N ≧). M, M>
N), a time constant τ is determined from the above curve by linear approximation for each rotation speed region.

When N ≧ H, τ = τ _H H> N ≧ M, τ = τ _M M> N, τ = τ _{L In} the case of radiating heat, τ = τ0 from the heat radiation curve obtained by experiment.
(Heat dissipation time constant).

Then, the calorific value Q obtained above and the time constant τ
The thermal displacement ΔL of the ball screw can be obtained from the following equation: ΔL = L · ρ · Q {1-exp (−t / τ)} (1) In the arithmetic processing, the first-order lag equation may be approximated using the following Runge-Kutta equation.

[0025] _{K 1 = Δt (u-x} ) / τ K 2 = Δt {u- (x + K 1/2)} / τ K 3 = Δt {u- (x + K 2/2)} / τ K 4 = Δt {U− (x + K ₃ )} / τ X = x + (K ₁ + 2K ₂ + 2K ₃ + K ₄ ) / 6 u: input x: output τ: time constant Δt: time interval The estimated value ΔL of the thermal displacement is The maximum value ΔLmax is provided, and the estimated value ΔL obtained by the calculation is the maximum value ΔLmax.
If x is exceeded, the estimated value ΔL is clamped to the maximum value ΔLmax. The correction calculation program is assumed to be started from the power-on time (the power-on time is defined as the initial time), ignores the influence of the initial temperature and the ambient temperature, and further corrects in a direction extending from the initial length at the initial time. In principle, no correction is made in the direction of shrinking.

The estimated value ΔL of the thermal displacement amount obtained as described above is uniformly distributed and added to the pitch error correction amount registered in the memory type pitch error correction function of the NC control device. Specifically, as shown in FIGS. 2 and 3, a correction area (X _{0 to} X _n :
In the case of the X-axis), the total amount of the estimated value ΔL is given to any of the-or + side area boundary points X ₀ and X _n of the correction area (X _{0 to} X _n ), and this is given. , Each point (X ₀ , X ₁ , X ₂ ,...) In the correction area (X _{0 to} X _n )
, X _n ) are linearly and uniformly distributed and added to the pitch error correction amount at each point. Note that the pitch error correction amount is usually registered in advance in the pitch error correction function of the NC control device, and the above points (X ₀ , X ₁ , X ₂ ,
.., X _n ) respectively correspond to the correction point coordinates of the pitch error correction function. Region origin X ₀ is the motor side according to the machine coordinate system is set to one of the anti-motor side. Figure 2 shows a case of setting the origin of the area X ₀ to + side of the X-coordinate system, FIG. 3 is a region origin X ₀ X coordinate system - shows the case of setting the side.

In the above description, the correction area (X _{0 to} X _n ) may be the entire effective length of the ball screw, or may be a specific area within the entire effective length. Generally, in a ball screw of a machine tool, an operation state in which a nut repeatedly reciprocates continuously over the entire effective length of a screw shaft is rare, and usually reciprocates within a specific region. Therefore, strictly speaking, the temperature rise distribution of the ball screw is not uniform over the entire effective length, in other words, the thermal displacement distribution is not uniform over the entire effective length, and the nut reciprocates in a specific region and other regions. There is uneven thermal displacement in the region. The idea of setting the correction area (X _{0 to} X _n ) to the entire effective length is that the entire ball screw is thermally regarded as one structure, and the estimated value ΔL is uniformly distributed to each point of the entire effective length. This has the advantage that simpler and quicker correction is possible. In addition, since there is actually heat conduction from a specific area to another area, the correction accuracy is sufficient. In particular, when the moving area of the nut with respect to the entire effective length is wide, a considerably high correction accuracy can be obtained. On the other hand, the idea of setting the correction area (X _{0 to} X _n ) as a specific area is based on the assumption that the specific area is regarded as a thermally independent area and the estimated value Δ
L is evenly distributed to each point in the specific area, and there is an advantage that more accurate correction is possible. Note that the estimated value ΔL in the latter case may be the total amount of the estimated value ΔL obtained as described above. By calculating the amount ΔL ′ to be allocated to the specific area, higher accuracy can be obtained (the remaining amount is allocated to another area). In this case, the correction calculation processing is executed independently for each of the specific area and the other areas, and the distribution calculation processing is distributed and added to the pitch error correction amounts corresponding to the respective areas. Further, the specific region is not limited to a single region, and may be plural. In this case, the allocation amount of the guess value ΔL is calculated for each specific region, the correction calculation process is independently executed for each specific region based on the allocation amount, and the pitch error correction amount corresponding to each region is calculated. Is distributed and added. These two
One example is that the total amount of the estimated value ΔL is non-linearly distributed and added to the pitch error correction amount.

Although the effects of the initial temperature and the ambient temperature are neglected in the above-described correction calculation, when these are taken into account, correction by a measurement cycle is also used. This is for determining the initial length of the ball screw. If an initial value (initial length of the ball screw) in a measurement cycle is input, the correction value ΔL calculated up to that time is canceled. Modify the pitch error correction function anew.

When the operation of the machine is interrupted (when the power is turned on again), the processing is performed as follows. First, the state of the machine when the power is turned on is roughly divided into two states: cold start and hot start. A cold start is a state in which the time from when the power is turned off to when the power is last turned on (power-off time: T) is equal to or longer than the heat radiation time constant (τ ₀ ) of the ball screw, and the machine temperature is adjusted to the ambient temperature. The hot start refers to a state in which the power-off time T is equal to or less than the heat radiation time constant τ ₀ and the machine temperature is not adjusted to the ambient temperature. With cold start and hot start,
The temperature rise curve of the first-order lag after the start of the correction calculation is different. Therefore, in order to distinguish between the cold start and the hot start, as shown in FIG. 4, the time t at which the correction control of the pitch error correction amount was performed is stored every moment, and the time t _{4 at} which the power was last turned off and the current power apply time is compared with the (power cycle time) t ₅ calculates a power-off time T. Then, in the case of a cold start (T> τ ₀ ), a calculation process of the estimated value ΔL is performed with the current power-on time t ₅ as an initial time,
In the case of a hot start (T <τ ₀ ), the processing of calculating the estimated value ΔL is performed using the previous power-on time t ₀ as the initial time again. Initial value of the estimated value ΔL, if the cold start zero, the value obtained by calculation from the values and the power interruption time T and the heat radiating time constant tau ₀ of ΔL at the previous power-off time t ₄ in the case of a hot start.

FIG. 5 conceptually shows an NC controller having a thermal displacement correcting function according to this embodiment. The thermal displacement calculator incorporated in the custom part of the NC controller receives the calculated value of N · q calculated by the calculator of the NC controller, and calculates the thermal displacement ΔL of the ball screw in the above-described manner. To process. Estimated thermal displacement ΔL calculated by the thermal displacement calculator
Is input to the pitch error correction function of the NC controller via the ball screw elongation correction function, distributed to each point of the pitch error correction amount registered in the pitch error correction function, and added. The corrected pitch error correction amount is output from the pitch error correction function to the servo amplifier of the servo control unit,
The rotation of the ball screw is controlled by the servo amplifier. When the measurement cycle is used together, the measurement program is built in the custom section, the initial length of the ball screw is measured in the measurement cycle, and output to the thermal displacement calculation section.

This NC control device has the following specifications.

(1) Forced reset and valid / invalid switching function Forced reset and valid / invalid switching can be performed by setting from the NC screen.

Forced reset The initial time, the calculated value, etc. are cleared, all are initialized, and the calculation is performed again from that point to start correction.

Effective / Invalid Switching It is possible to switch whether or not to perform correction based on the estimated value ΔL. (2) Screen display function of parameters and calculation results

Parameter Time constant τ: τH, τM, τL (sec) Control unit time Ts (sec) Sampling unit time ΔTs (sec) Coefficient: K Motor rotation threshold: H, M, L (rpm) Ball screw Overall length L (mm) Maximum value of estimated value ΔL ΔLmax (mm) Forced reset valid / invalid Calculation result Initial time Estimated value ΔL Pitch error correction amount after correction Motor rotation speed N Operating time ratio q (3) Data external output After correction Can be output to the outside.

[0036]

As described above, according to the present invention,
Since the thermal displacement of the ball screw is estimated using the first-order lag equation based on the motor rotation speed and the operating time ratio, there is no need to use special hardware when calculating the thermal displacement.
Thermal displacement correction can be performed by simple and quick arithmetic processing. In addition, a correction area to be corrected for the pitch error correction amount is set, and an estimated value of the thermal displacement amount is given to either the negative or positive side area boundary point of the correction area, and this is set in the correction area. Are linearly and uniformly distributed and added to each point, so that the arithmetic processing for correcting the thermal displacement can be simplified and speeded up.

The time constant τ in the equation of the first-order lag is obtained by dividing a curve representing the relationship between the rotation speed and the time constant obtained from the temperature rise curve of the ball screw into a plurality of rotation speed regions. by the obtained by linear approximation, the rotational speed
Without performing a complex analytical calculation constant τ time corresponding to,
It can be obtained relatively accurately.

When the operation of the machine is interrupted, the power-off time from the previous power-off time to the current power-on time is compared with the heat release time constant obtained from the heat release curve of the ball screw, and the cold start and hot start are determined. Correction accuracy can be improved by performing the computation process separately.

When the effects of the initial temperature and the ambient temperature are taken into consideration, the accuracy of the correction can be further improved by performing the above-described guessed value calculation processing based on the initial length of the ball screw measured by the measurement cycle. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing the number of rotations of a motor within a control unit time.

FIG. 2 is a diagram illustrating a method of distributing and adding an estimated value ΔL of a thermal displacement amount to a pitch error correction amount.

FIG. 3 is a diagram illustrating a method of distributing and adding an estimated value ΔL of a thermal displacement amount to a pitch error correction amount.

FIG. 4 is a diagram illustrating a relationship between a time t at which a pitch error correction amount correction control is performed during a machine operation (including interruption) and an estimated value ΔL of a thermal displacement amount.

FIG. 5 is a diagram showing an example of an N with a thermal displacement correction function according to this embodiment
It is a conceptual diagram which shows C control apparatus.

[Explanation of symbols]

 Ts Control unit time ΔTs Sampling unit time ΔL Thermal displacement T Power off time

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-240045 (JP, A) JP-A-62-288548 (JP, A) JP-A-5-277894 (JP, A) JP-A-6-88894 71541 (JP, A) JP-A-4-360753 (JP, A) JP-A-64-64750 (JP, A) JP-A-61-209853 (JP, A) Japanese Utility Model Laid-Open No. 61-201755 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) B23Q 15/00-15/28

Claims (3)

(57) [Claims] (1) Obtained from a temperature rise curve of a ball screw.
A curve representing the relationship between the rotation speed and the time constant
This curve is linearly approximated for each rotation speed region.
The time constant (τ) is determined, and the ratio of the number of rotations (N) of the motor per sampling unit time (ΔTs) in the control unit time (Ts) to the time during which the motor rotates within the control unit time (Ts) (Q:
From the operating time ratio), determine the calorific value (Q) of the ball screw, and correspond to the calorific value (Q) and the number of rotations (N) of the motor.
The estimated value (ΔL) of the thermal displacement of the ball screw is obtained from the first-order lag equation using the time constant (τ) described above, and the pitch error correction amount of the memory-type pitch error correction function of the NC controller should be corrected. A correction area is set, and the estimated value (ΔL) is given to either the negative or positive area boundary point of the correction area, and is linearly and uniformly applied to each point in the correction area. And correcting the pitch error correction amount by correcting the pitch error correction amount in the NC machine tool. 2. The time (t) at which the correction control of the pitch error correction amount is performed is stored every moment, and the power-off time (T) from the previous power-off time to the current power-on time and the heat radiation of the ball screw are stored. The heat radiation time constant (τ ₀ ) obtained from the curve is compared, and in the case of (T> τ ₀ ), the estimated value (ΔL) is calculated with the current power-on time as the initial time, and (T <τ _0), the thermal displacement correction method of the ball screw in the NC machine tool according to claim 1, wherein the performing the calculation of the estimated value ([Delta] L) again as the initial time of the last power-on time. 3. The ball screw initialization according to a measurement cycle.
The estimated value obtained by measuring the
3. The method for correcting thermal displacement of a ball screw in an NC machine tool according to claim 1, wherein (.DELTA.L) is canceled, and a calculation process of the estimated value (.DELTA.L) is newly performed based on the initial length. .